The present invention relates to an information recording medium for use in information recording devices such as hard disks and to the substrate thereof. More particularly, the invention relates to an information recording device which uses an information recording medium substrate wherein the surface recesses and protrusions have shapes within a given range and which thereby attains a higher information recording density and a higher information recording speed.
With remarkable progress in handling digital information in recent years, various devices for storing such information therein have been developed and produced. Improvements in these devices are ever-progressing, and the information recording capacity and the speed of recording/reproducing are increasing at an annual rate of tens of percents. Under these circumstances, the information recording devices which are currently used most widely are hard disks, and they are being improved at a higher rate than the other devices.
Hard disks have an information recording layer formed on a substrate for information recording media (hereinafter referred to simply as xe2x80x9csubstratexe2x80x9d), and information is recorded in this recording layer and reproduced with a magnetic head. At present, hard disks of the type called CSS or ramp road are generally used. In the CSS type, also called the contact-start-stop type, a magnetic head glides over the data zone of the disk when the substrate is rotating, and slides on the CSS zone of the substrate when the substrate stops and begins to rotate. The CSS zone of a substrate means that part of the substrate which has uniform recesses and protrusions which have been purposely formed and have a height on the order of tens of nanometers (such recesses and protrusions are formed mainly along the inner or outer periphery). In the ramp road type, a magnetic head glides over the substrate when the substrate is rotating, and is put in a housing position upon substrate stopping. This ramp road type does not necessitate a CSS zone because of the mechanism. In recent years, a hard disk called contact type is being investigated in which a magnetic head is always in contact with the substrate.
However, the conventional techniques have had the following problems. While the substrate of the CSS type or ramp road type is rotating, the magnetic head glides over the surface (information recording zone) of the substrate at a distance on the order of tens of nanometers (hereinafter, this distance is referred to as xe2x80x9cflying heightxe2x80x9d). It is therefore necessary to reduce the flying height for attaining a higher information recording density. However, in substrates having large surface recesses and protrusions, there is a higher possibility that the magnetic head might collide with protrusions on the substrate surface during rotation to cause head crushing. Even though the collision does not result in head crushing, there is a fear that the so-called thermal asperities may occur, in which the magnetic head detects abnormal signals and erroneously operates due to the heat resulting from the collision. In particular, since high-sensitivity MR heads or GMR heads have come to be mainly used recently, the problem concerning thermal asperities has become more serious.
It has been thought that the head crushing and thermal asperities attributable to collisions with protrusions present in the image recording zone can be avoided by reducing the sizes of the recesses and protrusions on that zone. Consequently, substrates having smaller surface recesses and protrusions have been regarded as substrates having higher performances. As a result of investigations made by the present inventors, however, it has been found that although reducing the sizes of the surface recesses and protrusions of a substrate surely reduces the frequency of head crushing, too small recesses and protrusions impair the flying stability of a magnetic head, rather than improving it, and result in an increased frequency of contacts between the magnetic head and the substrate surface and in a heightened possibility of head crushing. The reasons why the flying stability of a magnetic head thus decreases have not been elucidated. However, the present inventors presume that contacts of a magnetic head with a substrate are unavoidable because the head glides over the information recording zone while swaying, and that the smaller the surface recesses and protrusions present on the information recording zone, the higher the frictional force. The unavoidable contacts and the increased frictional force are thought to contribute to head crushing. Especially recently, considerably small flying heights are used for heightening recording density. It is therefore thought that the head crushing attributable to magnetic-head swaying has become more apt to occur.
Although information recording devices are required to attain an increase in recording capacity and an increase in the speed of recording/reproducing so as to cope with the trend toward handling of digital information as described above, they are strongly required to be more lightweight and smaller than conventional ones. In order for a substrate to satisfy these inconsistent requirements, it should have not only optimal shapes of surface recesses and protrusions but high rigidity and a high modulus of elasticity.
The invention has been achieved in view of the above-described problem noticed by the present inventors.
One object of the invention is to provide a substrate which has optimized shapes of surface recesses and protrusions and which therefore contributes to a reduction in flying height and causes neither head crushing nor thermal asperities.
Another object of the invention is to provide an information recording device which uses this substrate and attains a higher information recording density, a higher speed of recording/reproducing, and a smaller size.
These objects are accomplished with a substrate for information recording media which has surface recesses and protrusions wherein the height thereof at a contact proportion for the recesses and protrusions of 0.4% as determined with an atomic force microscope (AFM) is from 2 to 7 nm when the height of the recesses and protrusions at a contact proportion therefor of 50% is taken as a base.
In a preferred embodiment of the substrate of the invention described above, the height of the surface recesses and protrusions at a contact proportion therefor of 0.1% is from 2 to 10 nm.
In another preferred embodiment of the substrate of the invention described above, the protrusions in the surface recesses and protrusions have a diameter of from 10 to 1,000 nm.
Still another preferred embodiment of the substrate of the invention described above comprises a glass or crystallized glass as the base material.
The invention further provides a process for producing a substrate which comprises immersing a substrate comprising a glass or crystallized glass as the base material in an acid and an aqueous alkali solution to thereby form the shapes of surface recesses and protrusions described above.
The invention furthermore provides an information recording medium which comprises the substrate described above and, successively deposited thereon, at least an undercoat film, a magnetic film, and a protective film.
The invention still further provides a magnetic recording device employing the information recording medium described above.
A preferred embodiment of the information recording device of the invention described above is of the ramp road type.